Yeast cells arrest in the G1 phase of the cell cycle upon exposure to mating pheromones. As cells commit to a new cycle, G1 CDK activity (Cln/CDK) inhibits signaling through the mating MAPK cascade. Here we show that the target of this inhibition is Ste5, the MAPK cascade scaffold protein. Cln/CDK disrupts Ste5 membrane localization by phosphorylating a cluster of sites that flank a small, basic, membrane-binding motif in Ste5. Effective inhibition of Ste5 signaling requires multiple phosphorylation sites and a substantial accumulation of negative charge, which suggests that Ste5 acts as a sensor for high G1 CDK activity. Thus, Ste5 is an integration point for both external and internal signals. When Ste5 cannot be phosphorylated, pheromone triggers an aberrant arrest of cells outside G1 either in the presence or absence of the CDK-inhibitor protein Far1. These findings define a mechanism and physiological benefit of restricting antiproliferative signaling to G1.